On-treatment PLAtelet reactivity-guided Therapy modification FOR ST-segment elevation Myocardial infarction (PLATFORM) is an investigator-initiated, prospective, open-label, randomized, parallel-group, actively controlled single-centre clinical trial. The research protocol has been approved by the Ethics Committee of the Clinical Centre of Serbia. All participants will have to provide their informed consent in writing. The trial design will ensure that all participants abide by good clinical practice and the ethical principles of the Declaration of Helsinki II. Patients will be randomly allocated to antiplatelet regimen modification (ARM, interventional arm) or standard treatment (control arm) using a computer-generated 1:1 simple randomization scheme. TPR will be assessed in patients enrolled in the intervention arm of the trial. Low responders to aspirin will receive 200 mg aspirin for 30 days. Low responders to clopidogrel will receive 180 mg ticagrelor for 1 year. Patients enrolled in the control arm will receive standard antiplatelet regimen including 100 mg aspirin and 75 mg clopidogrel without assessment of TPR. The treating physicians will not be blinded to the intervention since an open design will make it possible for investigators to perform necessary adjustments of the antiplatelet regimen in accordance with TPR status. To minimize any possible bias inherent in the open design, endpoints will be evaluated by a blinded endpoint committee. Enrollment will start in June 2013. Recruitment will continue until 632 patients have been randomized. The end of the recruitment period is planned for June 2015. The trial will continue until all available survivors have been followed for at least 1 year.